1. Field of the Invention
The present invention relates to systems for controlling the power applied to a load, and in particular to a method to improve the performance of pulse width modulation (PWM) amplification.
2. Description of Related Art
For certain load control applications, it is desirable to have a high degree of precision in terms of linearity and power transfer efficiency. For instance, photolithographic systems require high resolution when in the scanning mode. Therefore, power transfer must be highly linear for controlling the positioning stages in the photolithographic systems, requiring that a discrete change in the position control signal to result in a proportional discrete change in the output signal for the positioning stages. At other times, photolithographic systems operating in a stepping mode require rapid changes in the positioning of the stages, which demand efficient power transfer to generate large acceleration and deceleration forces.
One of the highly effective methods of power delivery control is the use of pulse modulation amplifiers. They are used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors. A pulse modulation amplifier, such as a PWM amplifier, receives an analog waveform and outputs a series of square wave pulses. The square wave pulse has an amplitude and duration such that the integrated energy of the pulses is equivalent to the energy of the sampled input analog waveform multiplied by a gain factor created by the amplifier. The resulting PWM waveform may be filtered to produce an analog waveform replicating the original input waveform multiplied by the gain factor. The frequency of the desired sine wave is called the system frequency, while the frequency at which the switch operates is called the modulation frequency.
a. Prior Art Open-loop PWM
FIG. 1 is an example of a basic prior art PWM control circuit. The PWM controller 100 converts an analog input level Vs 110 into a variable duty cycle switch drive signal. As higher voltage output Vo 140 is required, the switch 120 is held on for a longer period. The switch 120 is usually both on and off once during each cycle of the switching frequency. As less voltage output is required, the duty cycle or percent of on time is reduced. A transistor operating as a switch dissipates no power when it is off and dissipates very little heat when it is on because of its low on-resistance. Other losses include heat generated in the flyback diode 150, which is small because the diode conducts only a very small portion of the time.
The inductor 130 stores energy during the on portion of the cycle for filtering. As a result, the load sees little of the modulating frequency but responds to the system frequency, which is significantly below the modulation frequency. With the PWM, the direct unfiltered amplifier output is either near the supply voltage or near zero. Continuously varying filtered output levels are achieved by changing only the duty cycle. For example, a high voltage requires the switch to be on longer than 50% of the time as shown in FIG. 2a; a medium voltage requires the switch to be on around 50% of the time as shown in FIG. 2b; and a low voltage requires the switch to be on less than 50% of the time as shown in FIG. 2c. 
As the duty cycle or the modulation frequency is increased and the polarity of the second half of the period is switched, the output square wave becomes more reflective of the sinusoidal input as shown in FIG. 3. The increase in modulation frequency also results in efficiency being quite constant as output power varies. However, there is a practical limit to higher frequency switching because of the limitation of the switching speed of the power switching devices, typically transistors.
b. Design Considerations of PWM
The challenge of designing a pulse width modulator is to get enough dynamic range to deliver the specified output while variables such as output current, input voltage, and temperature fluctuate over wide ranges. If output current remains constant, the average energy into the filter inductor must remain constant. As input voltage rises, the energy delivered to the inductor in a given time must be increased. If the input voltage is constant but output current decreases, less energy must be delivered to the inductor. The only variable the controller has to work with is the pulse width, which must be increased or decreased depending upon the load requirement. Therefore, PWM switching control is highly critical in determining the waveform output.
Furthermore, the design of the PWM has to take into considerations the following desired parameters: low internal losses to provide high operating efficiency, leading to small size and low cost equipment; high signal-to-noise ratio to provide quality power to the load; high modulation frequency to produce a variable frequency sine wave with minimum harmonics to minimize motor heating; and high surge ratings to protect against overcurrent and overvoltage conditions, thus improving reliability.
The main problem to resolve for all high-power amplifier and oscillator equipment is the removal of excess thermal energy produced in active devices, which can include switching resistance, diode forward drops, copper losses, and core losses. The temperature rise of the PWM circuit must be within the allowable limit as prescribed by the manufacturer of each component. The PWM circuit therefore must be designed to withstand worst-case internal power dissipation for considerable lengths of time in relationship to the thermal time constants of the heat sinking hardware. Consequently, the PWM circuit has to have the necessary heat dissipation device to cool itself under worst-case conditions, which include highest supply voltage, lowest load impedance, maximum ambient temperature, and lowest efficiency output level. In the case of reactive loads, maximum voltage-to-current phase angle or lowest power factor must also be addressed. The available cooling methods to remove the thermal generation include natural convection, forced convection, and conduction. If the excess thermal energy is not removed properly, the temperature rise can create circuit failure and/or reduce power delivery efficiency.
The other problem to be resolved is noise, or interference, which can be defined as undesirable electrical signals that distort or interfere with the original or desired signal. Examples of noise sources include thermal noise due to electron movement within the electrical circuits, electromagnetic interference due to electric and magnetic fluxes, and other transients that are often unpredictable. The main techniques used to reduce noise consist of applying shielding around signal wires, increasing the distance between the noise source and signal, decreasing the length that the desired signal must travel, and proper grounding of the entire system.
The ratio of the signal voltage to the noise voltage determines the strength of the signal in relation to the noise. This is called signal-to-noise ratio (SNR) and is important in assessing how well power is being delivered. The higher the SNR, the better the delivery of desired power. PWM amplification system with low SNR may not be suitable for photolithography motor drives and other high performance applications, which may require noise free power.
Further, conventional PWM amplifier systems do not provide drive current in a linear fashion and typically have poor total harmonic distortion (THD) characteristics. The THD and switching transients, which are associated with very high speed rising and falling edges, can cause noise and generate excessive undershoot and overshoot ringing effects. If these voltage spikes were allowed to exist they could cause high stress and possibly destruction of both amplifier and power supply components. To resolve the ringing effects, amplifier must use fast surge suppression to prevent ringing in the output signals.
The design challenges are compounded at higher frequency. As the modulation frequency is increased, a switching loss of each switching element is increased. In other words, use of high speed switching elements allows a decrease in switching loss but causes an increase in conduction loss. Further, pulse dropout occurs in a crest region and a zero-crossing region of the output signal waveform. Therefore, conversion efficiency as a whole is limited and system linearity is compromised.
c. PWM with Feedback Control
The open-loop PWM is adequate if precise control is not required and/or the load is constant. However, in most variable load control applications, a feedback signal is required to monitor if the power delivered matches the load and to adjust the modulating frequency. There are many forms of feedback control as well as variations of pulse modulating circuits as reflected by the numbers that are available in the market.
As described in U.S. Pat. No. 5,436,545, a pilot current detection circuit is used to sample and hold the average load current over a cycle at the midpoint of a first conduction state of a PWM amplifier. A correction circuit then generates a logic signal that measures the time of transition of the PWM amplifier from a second conduction state to an idle state. The logic signal is used to compare with the first sample and hold circuit to provide a scaling factor utilized to correct the average load current. Finally, the average load current and the scaling factor are combined in a multiplier to provide a scaled average load current to be fed back and control power delivered to the load. An error amplifier is used to integrate the difference between command signals and feedback signals. The PWM circuit converts the error amplifier output into a variable duty cycle drive signal, which varies from 0% to 100%. This scaled average load current approach is accurate if the signal has a perfect wave shape. However, in practice, the signal wave shape is superimposed and distorted by numerous static and dynamic noise signals, which may introduce the uncompensated errors in the averaging approach. The inherent problem is that only one sampled average value of the power delivered is taken and projected for compensation for the entire waveform(s).
Further, in the prior art, the rate of sampling and the speed of the feedback are inherently slower than the system overall frequency. This results in critical time delay between the error value, which was calculated from previous cycle(s), and the signal to be compensated which may have changed drastically in the mean time. Instead of requiring an addition of energy, the new signal may require a subtraction of energy. The untimely error feedback may increase the difference between the desired waveform and the actual waveform. As the speed of the system is increased, the error is magnified. Thus, conventional PWM amplifier systems are inadequate to produce the high degree of precision and power transfer efficiency required by current high performance motion control systems.
The system of the present invention overcomes the difficulties discussed above by calculating the excess or deficient energy, which is created by the aforementioned causes, and compensating for the error in the system as a whole. This is done by controlling the modulation frequency to generate the appropriate duty cycle, and modifying the immediate following pulse(s) based on calculated errors in the immediate preceding pulse. All of the errors, which may cancel each other out to a certain degree, are lumped together and treated as a collective whole. The invention compensates for the imperfections dynamically and almost instantaneously.
In one aspect of the present invention, the PWM system that delivers power to a load in response to an input signal comprises a pulse width modulator, a controller connected to the pulse width modulator, the controller controlling the pulse width modulator to generate a sequence of pulses in response to the input signal, and a detector connected to the controller, the detector determining value related to the error of the pulse based on the information of the pulse and its corresponding ideal pulse, wherein the controller makes the pulse width modulator generate compensated pulse by applying the value related to the error for at least one pulse to respective immediate following pulses in the sequence of pulses.
In one embodiment of the invention, the output energy of every pulse is measured or integrated on a pulse-to-pulse basis. The resulting integrated value of the pulse is subsequently digitized, and fed back to the digital controller. The controller then compares this information with the integrated value of an ideal pulse corresponding to the originally commanded signal, and compensates for any deviations, by adding or subtracting energy to or from the next pulse. By correcting each pulse, within the time frame of one pulse, this corrective action takes place at a frequency much higher than the system frequency. This being the case, the time it takes to make one correction, has little impact on the system response.
The controller can also detect and compensate for any repeating net energy losses, or increases, by treating these as a translation offset. This can also be done by error pattern recognition. One method is to observe the system manually and develop an algorithm. This is done by observing the input signal, the output signal, the difference between the two signals, and superimposing a mathematical equation onto the error portion of the signals. The repetitive errors, which occur in each and every pulse, end up appearing as DC terms in the output. They are only significant if they change over time, otherwise they can be compensated with an offset or a constant.
The final averaged energy of the pulses delivered to the load will thus have a higher accuracy when compared to the original commanded input. Errors from heat, noise, harmonics, ringing waveforms, imperfect square wave replication, and other static or dynamic distortion sources, will thus be compensated for, such that the net output energy will be an accurate representation of the commanded input.
In another aspect of the present invention, a stage device is disclosed which deploys a control system that includes the PWM system in accordance with the present invention. In a further aspect of the present invention, a lithography system is disclosed which deploys a stage device that incorporates the stage device in accordance with the present invention. In yet another aspect of the present invention, an object is formed by the lithography system in accordance with the present invention.